Acrylic polymer compositions have long been used as adhesive compositions, particularly in pressure-sensitive adhesive ("PSA") compositions. They exhibit adhesion to a broad variety of surfaces, show good low temperature performance, generally yield attractively clear adhesives and possess excellent heat, age and UV stability. Acrylic adhesive polymers are available as organic(solvent) solutions, aqueous emulsions and as 100 percent solids, thermoplastic systems that are melt applied. Perhaps initially most important were the solvent acrylics, but the environmental/health/safety needs of industry have resulted in dramatic growth in water based acrylics.
Traditionally acrylic PSA compositions have been tailored in characteristics by choice of acrylic monomers and polymerization conditions. This is unlike many other PSA compositions which traditionally have used tackifier resins, diluent oils, antioxidants, etc. to modify and enhance properties. With increasing desirability of water based systems and often their demonstration of significant performance enhancements with tackifier resins, increasing adhesive performance demands and a desire to minimize the need to adjust and modify the acrylics polymerization process, tackifier resins suitable for acrylics are being sought. Both rosin derivatives, such as rosin esters, and hydrocarbon resins are commercially used as tackifiers in such blends. The rosin esters have been preferred due to better compatibility with acrylics. However, the color and heat, age, and UV stability properties of rosin esters, even when hydrogenated, are inferior to those of acrylics and thus yield compositions of compromised properties when used as tackifiers for such systems. Hydrocarbon based resins have been employed as tackifiers for acrylics, but have often lacked the color, stability, compatibility or adhesive performance that are sought in high performance acrylic adhesive systems, particularly those based on the more polar acrylic adhesive polymers. Newer hydrocarbon resins suitable as tackifiers for acrylic copolymer compositions are thus being sought, as are acrylic copolymer compositions having improved properties including tack, adhesion, and clarity.
U.S. Pat. No. 5,106,902 addresses internally tackified acrylic copolymer compositions prepared by dissolving a hydrogenated hydrocarbon resin in a liquid monomer mixture from which the copolymer is to be prepared by free radical initiated emulsion polymerization. A preferred resin is a hydrogenated petroleum resin prepared by Friedel-Crafts polymerization of stem-cracked petroleum distillates, or fractions thereof, having boiling points between about 135.degree. C. and 220.degree. C. and containing 10-100% by weight vinyl aromatic monomer contents. Preferably the aromatic content of the resin is about 20-65%, most preferably 30-50%. The resins typically are said to have ring and ball softening points of between about 10.degree. C. and about 100.degree. C.
EP-B-0 196 844 describes the use in aqueous acrylic polymers of an aqueous emulsion of a hydrocarbon resin having a softening point from 10.degree. to 120.degree. C. and being a copolymer of predominantly C.sub.5 olefins and diolefins and from 10 to 60 wt % of one or more monovinyl aromatic compounds. A preferred range of the monovinyl aromatic compounds is 10 to 40 wt %, more preferably 10 to 30 wt %. The resin is not said to be hydrogenated and is lacking in compatibility with acrylic polymers containing butyl acrylate.
EP-B-O 388 497 describes a hydrocarbon resin tackifier for butyl acrylate based polymers wherein the resin is derived from a petroleum cracked distillate comprising unsaturated materials which are C.sub.5 -C.sub.6 mono-olefins and monovinyl aromatic monomers. These are used in proportions such that the resin has 35 to 85 wt % aromatic compound, preferably 40 to 70 wt %. The ting & ball softening point is 10.degree. to 90.degree. C., preferably up to 80.degree. C. This resin as well is not said to be hydrogenated. In the examples no softening point above 75.degree. C. is illustrated and % aromaticity is not greater than 80%.